Alkali Metal Ion-exchange in a Metal–Organic Framework Based on Lanthanum and 1,4-Phenylenebis(methylidyne)tetrakis(phosphonic acid)

Abstract

The ion-exchange selectivity of four metal–organic frameworks (denoted as MLaL), formed by alkali metal ions (M⁺), La³⁺, and 1,4-phenylenebis(methylidyne)tetrakis(phosphonic acid) (L), was examined. Unusual selectivity for the alkali metal ions was observed, which did not follow the previously proposed mechanism that was explained based on the ion-size similarity in the framework. The changes in the crystal structures after ion-exchange reactions were observed by powder X-ray diffraction analysis. The change in the lattice energy in a mixed-metal framework is likely to be one of the significant parameters to affect ion-exchange selectivity.